Hydrocarbon gas liquefaction by admixed gas-liquid expansion and heat exchange



Aug. 27, 1968 J. L. HORTON 3,398,543

HYDROCARBON GAS LIQUEFACTION BY ADMIXED GAS-LIQUID EXPANSION AND HEATEXCHANGE Filed March 23, 1966 FUEL 64.5 0117' 34 RAW 645 W RICH ETHANOLINVENTOR. JOHN LEROY HORTON A TTORNE Y United States Patent 3,398,543HYDROCARBON GAS LIQUEFACTION BY AD- MIXED GAS-LIQUID EXPANSION AND HEATEXCHANGE John Leroy Horton, Shreveport, La., assignor to American glachine & Foundry Company, a corporation of New erseyContinuation-impart of application Ser. No. 306,428, Sept. 4, 1963. Thisapplication Mar. 23, 1966, Ser. No. 536,911

3 Claims. (Cl. 62-11) ABSTRACT OF THE DISCLOSURE A hydrocarbon feed gasis separated by cooling the feed gas in the intermediate passage of aconcentric three passage heat exchanger to condense a portion of thehydrocarbons to form a lighter gaseous component and a heavier liquidcomponent. A separated expanded gaseous component and an expandedgas-liquid admixture is passed in heat exchange with the feed gas in theconcentric heat exchanger without permitting liquid to accumulate in aseparation zone.

The present application is a continuation-in-part of application Ser.No. 306,428, now abandoned, filed on Sept. 4, 1963.

This invention relates to gas sepaartion, and more particularly relatesto a method and apparatus for separating various components of a streamof vapors by regenerative cooling thereof under substantiallyconstant-enthalpy conditions.

It is a principal object of this invention to provide a method andapparatus in which a high degree of etficiency results from pre-cooling,separating and then expanding the separated components of the inputstream and feeding back substantially all of the recovered components inseparate paths and in indirect heat-exchange relationship with theincoming stream, thereby cooling the latter and superheating theseparated components in each feedback path.

It is another object of the invention to provide a coaxial system whichcan be conveniently heat-insulated to prevent transfer of ambient heatthereinto.

Another major object of this invention is to provide an improved methodand apparatus for stripping heavier components from the input vaporstream using improved expansion and regeneration techniques. The coolingachieved due to the Joule-Thomson eifect, by expansion of the separatedcomponents and regenerative counter-flowing thereof, is many times asgreat as would be obtained by expansion alone. This improvement resultsessentially from accumulating the temperature drop by feeding backsubstantially all separated and cooled components so that the weight offlow through the apparatus is essentially the same both in the forwardand in the regenerative directions. The degree of heat exchange in theregenerative path being designed so that it is adequate to againsuperheat the separated component streams by efliciently absorbing heatfrom the incoming stream, ambient atmospheric heat being essentiallyexcluded.

Another object of the invention is to provide a method and apparatuswherein more of the heavier components are recovered as a result ofseparation at a relatively high pressure and before expansion, andwherein the pressure in the separator is maintained below the retrogradepressure of the cooled vapors. This is critical because separation at apressure above the retrograde point would result in a carry-over ofheavy components as vapors in the device.

Other objects and advantages of the invention will be- "ice comeapparent during the following discussion of the drawing showing a flowdiagram of a practical embodiment of the invention.

The embodiment shown in the drawing illustrates a convenient arrangementof two heat-exchangers 10 and 20 both of which comprise three concentrictubes through which fluid streams flow. Exchanger 10 includes a smallinner tube 12, an intermediate tube 14, and an outer tube 16. Anexternal layer of heat insulating material 18 is preferably applied tothe outer periphery of tube 16. The heat exchanger 20 is of similarconstruction and includes an inner tube 21, an intermediate tube 24, andan outer tube 26, and heat insulation 28.

In both exchangers there are three flow paths. There is an inner pathcommunicating with the bores 11 and 21 of inner tubes 12 and 22respectively, via the pipe 32, and communicating at one end with a richgas outlet pipe 31 and at the other with the expansion valve 46 via thepipe 33 from separator 40. There is also an intermediate path throughtubes 14 and 24 wherein the raw inlet gas proceeds to the separator 40.The inlet gas enters the system through the pipe 34, passes through theannulus 13 between the inner tube 12 and the intermediate tube 14, andflows through the connecting pipe 35 into a water knockout 36. From theknock-out unit 36 the gas flows through the annulus 23'inside of heatexchanger 20 and passes via pipe 38 to the separator 40.

The outer path of the system includes the pipe 41 leading from separator40, the annulus 25 between the intermediate tube 24 and the outer tube26, the pipe 42, the annulus 15 between the intermediate tube 14 and theouter tube 16, and the gas outlet pipe 43. The intermediate pathreceives and conduct the relatively hot and high pressure inlet gas W tothe separator 40 and is the main, or forward-flow path; whereas theinner and outer paths are counter-flow regenerative paths, deliveringfuel gas F and rich gas R at their outlets 43 and 31 respectively.

In this system of the invention hereinabove described, it is importantthat the counter-flow regenerative paths exiting at 31 and 43, bearranged as shown with respect to the incoming stream as thisarrangement provides the maximum drive for the heat transferrelationship and results in a lower heat gain from the ambient air. Italso exposes the least amount of heat transfer surface in theexchangers. In fact, the proper ratio of the area of heat transfersurface, labeled 14, 12, 22 and 24 must be maintained or the temperatureprofiles producing heat transfer Will not be proper for efiicientregeneration. Lacking this proper balance, continuity of the process ishampered and efiiciency becomes impractical.

OPERATION In the system of the invention as depicted, the raw inlet gasW is introduced through the intermediate concentric inlet pipe 34 andprogresses in the direction of the separator 40. Due to cooling of theraw gas stream accomplished in the two heat exchangers, the temperatureof the stream will be substantially lowered by the time it reaches theseparator 40, thereby condensing a fraction of the feed stream. Theseparator 40 divides the cooled stream into two component streams,including a lighter or vapor stream which passes from the separator 40through pipe 44, and a heavier stream which passes from separator 40through the pipe 39.

In accordance with the invention it has been found that separator 40must operate in a manner which permits no liquid to accumulate inseparator 40. Operation is conducted so that all the liquid separated inthe separator 40, as well as a minor portion of the vapor separatedtherein, passes through and is educte-d through pipe 39 and valve 46.The adjustment of valve 46 and the operation of the separator in themanner described is essential for regeneration to cold, i.e., sub-zerotemperatures to occur.

The gaseous stream leaving through the pipe 44 is expanded by anautomatic pressure reducing regulator 45 acting as a throttling valvethereby cooling the vapors due to the Joule-Thomson effect. Thecomponent mixture of liquid and small amount of vapor passing outwardlythrough the pipe 39 are also expanded and cooled substantially followingpassage through valve 46.

The amount of cooling across valve 46 is dependent upon the amount ofvapor expanded with the liquid across the valve and varies directly withthe vapor expanded. The greater the volume of vapor expanded with theliquid, the greater the temperature drop across this valve.

The expanded and chilled components R and F are then respectivelyconducted through the inner and outer counterfiow paths in the two heatexchangers l and 20 for the purpose of regeneratively absorbing heatfrom the incoming mainfiow gas stream W in the intermediate path. Awater knock-out 36 is preferably provided between the two heatexchangers and in the mainfiow path of the incoming gas for the purposeof removing any water, and undesirable constituents which condense fromthe feed stream as a result of the cooling in the first heat exchanger10.

The water knock-out 36 is preferably provided with a bleed arrangementcomprising a cylinder 53 open at both ends. Water removed at 48 is theonly component of the input gas W which is not regenerativelycounterflowed. The top of the outlet pipe 54 is somewhat elevated insidethe cylinder 53, so that as the upper hydrocarbon layer and the lowerwater layer accumulate, water is drained continuously. As more wateraccumulates it tends to raise the head outside the cylinder 53 which inturn forces more water out of pipe 54 resulting in essentiallymaintaining an equilibrium level. The effect of removal of minorquantities of water at this point does not significantly effect theWeight of the product counterflow since it is only a very smallproportion of the total vapor content.

Although an insulating construction is illustrated only at 18 and 28 onheat exchangers 10 and 20, it will be understood that the various otherpipes including the water knock-out 36 and the separator 40, preferablyshould be well heat insulated on their external surfaces. This isnecessary to prevent, to as great an extent as possible, the absorbingof ambient heat into the system.

To avoid interruption of operation, since the temperature achieved inseparator and exchanger 20 is below the hydrate point of the mainstream, freezing in passage 23 and in the separator 40 should beprovided against. This is easily accomplished by the injection ofsuitable quantities of methanol or other suitable antifreeze into pipe37. An antifreeze tank 50 with a differential pressure line 51conveniently connected to feed line 34 help to force the antifreeze intoline 37 aided by valve 52. The rate of methanol flow may be adjusted bychanging the position of valve 49.

Except for a small amount of water drained from the pipe 48, the flow ofraw vapor W into the heat exchangers 10 and 20 through the pipes 34 and37 precisely equals the counterfiow in the regenerative direction, thiscounterflow being obtained by operating into a lower pressure at theoutlets 31 and 43 as compared with the pressure at the inlet 34. In manyprior art systems, one or more of the separated hydrocarbon componentsis delivered to storage Without any expansion and/ or introductionthereof into regenerative flow. The degree of cooling due to theJoule-Thomson effect is made to multiply in the present system byexpansion of all of the components after their mutual separation, and byfeeding back the expanded components in heat-exchange relation with theincoming raw gas. These two features are combined in a system operatingwith such a relatively high degree of heat insulation that there is verylittle difference between the temperatures of the entering and leavingstreams. So much heat is absorbed from the incoming stream W by thedeparting streams F and R in the heat exchangers 10 and 20 that the fuelgas F discharged at the pipe 43 and the rich gas R discharged at thepipe 31 are'both discharged in su erheated condition at very nearly thesame temperature as the temperature at which they were introduced intothe system at the raw gas inlet pipe 34, but at a considerably lowerpressure. It is significant to note that the separation of thecomponents must occur prior to reduction in pressure at the expansionvalves 45 and 46. Also it is important that separation is carried out ata higher pressure because this encourages a greater separation of theheavier components as long as the upper limit of pressure is below theretrograde point.

As noted hereinabove, it is necessary in operation of the system thatvalve assembly 46 be operated in a manner permitting all of the liquid,which is isolated in the separator, plus some vapor to be throttled,resulting in the maximum temperature drop across the valve. Repeatedattempts to attain the desired regeneration with the valve passingliquid only were unsuccessful.

The following table of temperatures and pressures, taken at the variouspoints of the system labeled on the diagram, complies the results ofactual tests run on apparatus of the type shown in the illustrateddrawing.

Data Series A B 0 Wet gas volume (M. c.f.d.) 115 0 112. 0 95. 2 Rich gasvolume (M. c.f.d.) 62. 4 61. 8 46. 0 Fuel gas volume (M. c.f.d) 52 6 50.2 49 2 Raw gas into exchanger 10 100 98 96 Raw gas into exchanger 20 F.)45 43 48 Raw gas into separator F.) 25 30 20 Fuel gas to exchanger 20-50 45 Fuel gas to exchanger 10 F.) l0 8 10 Fuel gas out F.) 62 58 Richgas to exchanger 20 F.) -78 78 --78 Rich gas to exchanger 10 F.) -2 -2-2 Rich gas out F.) 66 61 Wet gas inlet pressure (lbs.) 287 285 287Separator pressure (lbs)... 287 285 287 Fuel gas pressure (outlet) (lbs40. 5 40. 5 40. 5 Rich gas pressure (outlet) (lbs.) 6. 0 5. 0 1. 5Ambient air temperature F.) 103 99 87 Defining the degree ofregeneration as the ratio of the temperature drop in the wet gas streambetween the inlet and the separator 40 as compared with the temperaturedrop of the fuel gas across the valve 45, calculations made from theabove test data show that the degree of regeneration for series A, B,and C, respectively, was 5.00, 5.12 and 4.63.

The present invention is not to be limited to the exact form illustratedin the drawing, in view of the fact that various changes apparent to oneskilled in the art may be made within the scope of the following claims.

I claim:

1. The method of separating a vaporous feed mixture of hydrocarbonswhich is introduced at a relatively higher pressure into gaseous andliquid components at a relatively lower pressure, including the stepsof:

(a) flowing the mixture in the intermediate passage of a concentricthree passage heat-exchanger;

(b) separating the hydrocarbons into a heavier liquid component and alighter gaseous component in a separation zone;

(c) expanding a part of the lighter gaseous component to provide a firstproduct stream;

(d) Without permitting liquid to accumulate in the separation zoneeducting in admixture the heavier liquid component withdrawn from theseparation zone with the remaining part of the lighter gaseous componentand expanding the heavier liquid component in admixture with theremaining part of the lighter gaseous component to provide a secondproduct stream; and

(e) counterfiowing said expanded streams, which comprise essentially theentire weight of said feed mixture, in regenerative heat-exchangerelationship with said mixture to chill the latter While heating theformer by flowing one of said streams through the inner passage of saidthree passage heat exchanger and the other stream through the outerpassage thereof;

(-f) substantially excluding ambient heat to restore the temperature ofthe expanded components after counterfiow to substantially the sametemperature of the mainflow hydrocarbon feed mixture.

2. The method of claim 1, wherein the lighter gaseous component flowsthrough the outer concentric passage of said heat exchanger.

3. The method of claim 1, wherein an antifreeze is introduced into thevaporous mixture prior to the sepa ration.

References Cited UNITED STATES PATENTS Pollitzer 6220 Moore 62-11 Coffey23212 Keyes 6214 XR Horton et al 6220 Moon et al 62-31 XR Huddleston6223 Harper et al. 6223 De Lano 6223 OTHER REFERENCES Fuels and TheirCombustion by Haslam & Russell,

Economic Aspects of Producing Pipeline Gas, Using Lurgi Generators forGasifying Bituminous Char by Katell, Faber & Constantine, 1959, Bureauof Mines NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner.

